The purpose of this project is to study the mechanisms responsible for impulse initiation in carotid body chemoreceptors. Several approaches have been used: 1) neurochemical studies which have shown that acetylcholine is contained in glomus type I cells and that this agent is released during carotid body stimulation induced by hypoxia and hypercapnia. These observations have been accomplished by using autoradiography and high voltage electrophoresis of carotid bodies and of the runout of Locke's solution bathing the carotid body at rest and during stimulation. 2) The role of the glomus type I cells in receptor function has been investigated by using tissue slices and Nomarski optics. In this fashion the glomus cells have been visually identified in the living tissue slices and have been penetrated with microelectrodes. During stimulation of the cells by NaCN, hypoxia, etc., the cells undergo hyperpolarization while the nerve endings become depolarized. These observations seem to rule out a direct electrical effect of cell membrane potential changes.